1. Field of the Invention
The present invention generally relates to the regulation of power converters and more particularly to the regulation of amplifiers using remote sensing to control the signal at the amplifier load.
2. Description of the Related Art
It is often desired to improve the regulation of a voltage source at its load by sensing and controlling the actual voltage produced at the load. Such remote sensing, as it is called, overcomes limitations of finite source impedance which derives from the impedance of connecting cables, connectors, filters, etc., that lie between a signal source and a load. In the simplest of situations, one would derive all controlling feedback from the voltage induced across the load.
In practice, this does not allow for a wide bandwidth closed-loop system, as the impedances between the source and the load form a low-pass filter which adds phase lag to the feedback and thus does not improve the closed-loop stability of the system.
This filter/stability problem may be addressed by deriving the high-frequency feedback signal local to the signal source and remotely deriving the low-frequency feedback signal at the load.
A simple circuit is shown in FIG. 1 which automatically aligns the high and low-pass characteristics of the system. This approach may be used on DC-to-DC power supplies and AC power/signal sources as well. Conventional AC line voltage generators use remote sensing to improve the accuracy of the voltage delivered to the load (i.e., the device under power).
Some audio amplifiers use remote sensing to achieve very high damping factors at their output terminals by eliminating the internal impedance added by internal filters and termination impedances. In the latter case, where an accurate frequency response is required, it is possible to correct a small response error created by the output loading of the remote sensing network R1,C1 by adding R2,C2 to the input network as shown in FIG. 1.
Switch-mode amplifiers have output filters which create a significant output impedance and phase shift. If all feedback is taken at the output of the filter, very little high frequency voltage feedback will be possible, especially if the load is an arbitrary impedance. If all voltage feedback is taken at the input to the filter, on the other hand, the feedback signal will include large amounts of pulse width modulation (PWM) ripple which will create distortion if introduced into the control loop. A filter having large attenuation in the frequency bands corresponding to the frequency of the switching ripple is required to reject the PWM spectra. Similar issues are associated with current feedback signals.
The local or high-pass portion of the voltage feedback may be designed to use either the ripple attenuating properties of the output filter or to augment that filter with its own ripple reducing attributes. A continuum of solutions is possible where some or none of the output filter is used for the local high-pass portion of the feedback. If none of the output filter is used, then maximum stability margins can be obtained as the load impedance changes. Any load regulation improvement that occurs in such a system must come through the low-pass feedback path. It is desirable to use the highest cross-over frequency possible to maximize the output regulation.
The performance of this system can be jeopardized when the series impedance between the actual signal source and the load acts as a high-Q inductance and the load contains a high-Q shunt capacitance. The series resonant network formed by this common configuration results in a large peak gain at the resonant frequency which can easily equal or exceed the attenuation of the low-pass filter formed for the load feedback. Instability may result when the gain margin of the feedback loop is overwhelmed by the gain peak of this resonance. This condition is common to both switch-mode amplifiers and AC line voltage sources. A switch-mode amplifier used as a gradient amplifier has a resonance form with the inductance of its output filter and the added capacitance of electromagnetic interference (EMI) filters on the gradient cables. A line voltage generator must power electronic power supplies with large input filters. In both cases, the Q of the inductances must be high or conduction losses will be severe. Also, the Q of the capacitances is often high to allow maximum EMI filtering.
If the circuit of FIG. 1 is used, the cross-over frequency between the load and local feedback must be set to a lower frequency than ideal, reducing the load regulation bandwidth. What is needed is more attenuation of the high frequencies fed back from the load without a serious reduction in feedback at frequencies well below resonance.
The present invention provides an amplifier having a remote sensing system using a high-order filter to combine a remote sensing low-pass signal and a local high-pass feedback signal. The amplifier includes a high-pass feedback signal, a remote sensing low-pass signal, and a constant-sum filter combining the high-pass feedback signal and the remote sensing low-pass signal. The constant-sum filter is at least a second order filter and generates the voltage feedback control signal. In one embodiment of the invention, the constant-sum filter is a three-terminal, low-pass filter with unity gain having an input terminal, an output terminal, and a common terminal. The input terminal receives the remote sensing low-pass signal. The common terminal receives the high-pass feedback signal. The output terminal transmits the feedback signal.